Current-limiting devices are critical in electric power transmission and distribution systems. For various reasons, such as a lightning strike, a short circuit condition can develop in a section of a power grid causing a sharp surge in current. If this surge of current, which is often referred to as fault current, exceeds the protective capabilities of the switchgear equipment deployed throughout the grid system, it could cause catastrophic damage to the grid equipment and the customer loads that are connected to the system.
Superconductors, especially high-temperature superconducting (HTS) materials, are well suited for use in a current-limiting device because of their intrinsic properties that can be manipulated to achieve the effect of “variable-impedance” under certain operating conditions. A superconductor, when operated within a certain temperature and external magnetic field range (i.e., the “critical temperature” (Tc) and “critical magnetic field (Hc) range), exhibits no electrical resistance if the current flowing through it is below a certain threshold (i.e., the “critical current level (Ic)), and is therefore said to be in a “superconducting state”. However, if the current exceeds this critical current level, the superconductor will undergo a transition from its superconducting state to a “normal resistive state”. This transition of a superconductor from its superconducting state to normal resistive state is termed “quenching”. Quenching can occur if any one or any combination of the three factors, namely the operating temperature, external magnetic field or current level, exceeds the corresponding critical level. Mechanisms, using any one or a combination of these three factors, to induce and/or force a superconductor to quench, is usually referred to as a trigger mechanism.
A superconductor, once quenched, can be brought back to its superconducting state by bringing the operating environment to within the boundaries of its critical current, critical temperature and critical magnetic field range, provided that no thermal or structural damage was done during the quenching of the superconductor. HTS material can operate near the liquid nitrogen temperature (77° K) as compared with low-temperature superconducting (LTS) material that operates near liquid helium temperature (4° K). Manipulating properties of HTS material is thus much easier because of its higher and broader operating temperature range.
For some HTS materials, such as bulk BSCCO, YBCO and MgB2, there often exists within the volume of the superconductor non-uniform regions resulting from the manufacturing process. Such non-uniform regions can develop into the so-called “hot spots” during the surge of current that exceeds the critical current level of the superconductor. Essentially, at the initial stage of quenching by the current, some regions of the superconductor volume become resistive before others do due to non-uniformity. A resistive region will generate heat at these non-uniform regions from its associated i2r loss. If the heat generated could not be propagated to its surrounding regions and environment quickly enough, the localized heating will damage the superconductor and could lead to the breakdown (burn-out) of the entire superconductor element.
Additionally, these non-uniform hot spot regions can inhibit recovery transition of the superconducting material under load after a fault current pulse. Specifically, during a transition under load, these hot areas (or hot spots) may remain in normal resistive state indefinitely, dissipating heat under the load current. Thus, the non-uniform local variations in critical current and heat transfer (e.g., due to the formation of nitrogen bubbles) along the conductor make designing a fault current-limiter with fast recovery under load capabilities challenging.